Herein the expression “catalyst composition” denotes a catalyst, or more than one catalyst, or a catalyst system useful for catalyzing a polymerization reaction.
Advances in polymerization and catalysis have resulted in the capability to produce many new polymers having improved physical and chemical properties useful in a wide variety of superior products and applications. With the development of new catalysts the choice of polymerization process for producing a particular polymer has been greatly expanded. Also, advances in polymerization technology have provided more efficient, highly productive, and economically enhanced processes. Illustrative of these advances is the development of technology utilizing metallocene catalyst systems and other advanced metallocene-type catalyst systems.
It is commercially important to produce “bimodal” polymers. A bimodal polymer has at least one low molecular weight fraction and at least one high molecular weight fraction, and a molecular weight distribution that determines an identifiable (relatively high) molecular weight for the high molecular weight fraction and another identifiable (relatively low) molecular weight for the low molecular weight fraction. A multimodal polymer has at least two (e.g., two, three or more) molecular weight fractions, and a molecular weight distribution that determines a different identifiable molecular weight each molecular weight fraction. In the present disclosure, the expression “bimodal polymer” denotes any polymer in the broad class of multimodal polymers, regardless of whether the polymer has only two or more than two molecular weight fractions.
The molecular weight distribution of a bimodal polymer is sometimes described by the ratio of weight average molecular weight of a high molecular weight fraction thereof to weight average molecular weight of a low molecular weight fraction thereof. Another parameter sometimes used to describe the molecular weight distribution of a bimodal polymer (or of each molecular weight fraction of such a polymer) is the polydispersity index Mw/Mn, where Mw denotes weight average molecular weight and Mn denotes number average molecular weight. Each molecular weight fraction of a bimodal polymer typically has a different density.
It would be desirable to control the activation of at least one of the catalyst components (i.e., a number, M, of the catalyst components, where M is not less than one) of a polymerization catalyst composition (e.g., a polymerization catalyst system) including N catalyst components (where N is a number not less than two, and N>M), during a polymerization reaction (catalyzed by the catalyst or catalyst system) for producing a bimodal polymer, to control the polymer's molecular weight distribution.
Herein, the expression “bimodal catalyst composition” (or “bimodal catalyst system”) denotes a catalyst composition (or catalyst system) useful to catalyze a polymerization reaction to produce a bimodal polymer. A bimodal catalyst composition includes at least two catalyst compounds: at least one (sometimes referred to herein as an “HMW catalyst”) for catalyzing polymerization of a high molecular weight fraction of the product and at least one (sometimes referred to herein as an “LMW catalyst”) for catalyzing polymerization of a low molecular weight fraction of the product.
Typical polymerization reactions in a fluidized bed gas phase reactor employ a continuous cycle. In one part of the cycle, a cycling gas stream (sometimes referred to as a recycle stream or fluidizing medium) is heated in the reactor by the heat of polymerization. This heat is removed from the recycle stream in another part of the cycle by a cooling system external to the reactor. Generally, in a gas fluidized bed process for producing a polymer product, the recycle stream is a primarily gaseous stream containing one or more monomers that is continuously cycled through the fluidized bed in the presence of a catalyst under reactive conditions. The recycle stream is withdrawn from the fluidized bed and (after cooling) is recycled back into the reactor. Simultaneously, polymer product is withdrawn from the reactor and fresh monomer is added to replace the polymerized monomer.
In some conventional polymerization reactions, a fluidized bed gas phase reactor system operates in a “condensed mode” (see, e.g., WO 2007/030915) in which the recycle stream is cooled to a temperature below the dew point in the reactor. Typically, this is accomplished by causing at least one induced condensing agent (“ICA”), in an appropriate concentration or concentrations, to be included in the recycle stream (and by controlling the recycle stream temperature) so that a portion of the recycle gas stream condenses. The resulting recycle stream contains entrained liquid.